selftests: netfilter: synproxy test requires nf_conntrack

Otherwise, this test does not find the sysctl entry in place:

 sysctl: cannot stat /proc/sys/net/netfilter/nf_conntrack_tcp_loose: No such file or directory
 iperf3: error - unable to send control message: Bad file descriptor
 FAIL: iperf3 returned an error

Fixes: 7152303c

 ("selftests: netfilter: add synproxy test")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>